Phenol-oxidizing peroxidases contribute to the protection of plants from ultraviolet radiation stress

We have studied the mechanism of UV protection in two duckweed species (Lem naceae) by exploiting the UV sensitivity of photosystem II as an in situ se nsor for radiation stress, A W-tolerant Spirodela punctata G.F.W. Meyer eco type had significantly higher indole-3-acetic acid (IAA) levels than a UV-s ensitive ecotype. Parallel work on Lemna gibba mutants suggested that UV to lerance is linked to IAA degradation rather than to levels of free or conju gated IAA. This linkage is consistent with a role for class iii phenolic pe roxidases, which have been implicated both in the degradation of IAA and th e cross-linking of various UV-absorbing phenolics. Biochemical analysis rev ealed increased activity of a specific peroxidase isozyme in both UV-tolera nt duckweed Lines. The hypothesis that peroxidases play a role in UV protec tion was tested in a direct manner using genetically modified tobacco (Nico tiana sylvestris). It was found that increased activity of the anionic pero xidase correlated with increased tolerance to UV radiation as well as decre ased levels of free auxin. We conclude that phenol-oxidizing peroxidases co ncurrently contribute to UV protection as well as the control of leaf and p lant architecture.